In the field of geophysical prospecting, seismic signals are used to do 3-D seismic surveys of a predetermined area. However, problems arise in the collection of data due to the backscattering of energy from the shallow inhomogeneities or obstacles.
Prior Art at Compagnie Generale de Geophysique:
Compagnie Generale de Geophysique utilizes a process called Deterministic Diffractor Noise Reduction (DDNR) to remove the contribution of diffractor energy from the survey. DDNR involves identifying and picking travel times for each diffractor located at sea bottom. Once travel times are known for a diffractor, then its location can be calculated by assuming a speed of propagation, like 1500 m/s, for the medium. Data can then be flattened using travel times calculated for the diffractor and the flat component of energy (diffraction) can be attenuated using FK filter or Radon transform filter, as it is known in the art.
Prior Art in the Industry:
Fookes et al. (“Practical interference noise elimination in modern marine data processing,” Expanded Abstracts, 2003 SEG Annual Meeting) follow a method similar to Compagnie Generale de Geophysique DDNR method mentioned above: pick travel times and find the diffractor location that minimizes the error between calculated travel times and measured travel times. Upon determination of diffractor location, the data is flattened and flat events suppressed.
A use of coherency measurement is using semblance in stacking velocity analysis of seismic data and is done by Taner and Koehler (1969, “Velocity spectra—digital computer derivation and applications of velocity functions,” Geophysics, 34, 859-881). The use of other coherency measures than semblance for lateral coherency of events is also possible: energy normalized cross correlation sum, stacking power, or stacking amplitude are other possibilities. Key and Smithson (1990, “New approach to seismic-reflection event detection and velocity determination”: GEOPHYSICS, Soc. of Expl. Geophys., 55, 1057-1069.) derived their coherency measure from the eigenvalues of the problem at hand. Gulunay (1991, “High resolution CVS: Generalized covariance measure”, 61st Ann. Internat. Mtg: Soc. of Expl. Geophys., 1264-1267) studied the relationship of such coherency measures including the ones derived from semblance. Gonzalez-Serrano and Chon (“Migration velocity analysis in 3-D,” Expanded Abstracts, 1984 SEG Annual Meeting), Sicking (“Diffraction semblance for velocity and structure analysis,” Expanded Abstracts, 1987 SEG Annual Meeting), and VarWest et al. (“Relation between velocity fields and imaging in the presence of lateral velocity variations,” Expanded Abstracts, 1985 SEG Annual Meeting) use semblance analysis for prestack migration velocity determination. Landa et al. (“A method for detection of diffracted waves on common offset sections,” Geophysical Prospecting, 35, 359-373, 1987) use semblance analysis to find buried edges in x-z plane that cause diffraction under a 2-D seismic line (shooting and receiving along x direction, shots and receivers at the surface, z=0) using common offset data (x-t). Landa and Keydar (“Seismic monitoring of diffraction images for detection of local heterogeneities,” Geophysics, 63, 3, 1093-1100, May 1998) use semblance analysis to detect local heterogeneities (diffractors) buried under a 2-D section (x-z plane) using a source receiver configuration similar to the ones used in 2-D seismic recordings (shooting and receiving along x-z direction, shots and receivers at the surface, z=0). The paper discusses a “D-section” which is similar in concept to semblance scanning; however, D-section is done for diffractor buried in a vertical plane of a complex earth. U.S. Pat. Nos. 6,687,618 and 6,546,339 also address the use of semblance scan in geophysical processing using seismic signals.
Two papers by Blonk et al (1994, “Inverse scattering of surface waves: A new look at surface consistency”, Geophysics, 59, 6, 963-972 and 1995, “An elastodynamic inverse scattering method for removing scattered surface waves from field data”, Geophysics, 60, 6, 1897-1903.) address the issue of finding and removing such diffractors for land data but their method is based on “linearized elastodynamic inverse scattering theory” and involves consideration of temporal frequency, solution of linear systems with tools like conjugate gradient algorithm and is completely different from the time domain amplitude coherency approach of the arrival energy used in this invention.
It is an object of this invention to remove the energy from the survey that is contributed by the diffractor.
It is a further object of the present invention to eliminate the task or necessity of picking of arrival or travel-times or going into complex theoretical calculations as in Blonk et al (1994,1995) papers for determination of diffractor locations.